There have been numerous attempts to develop intervertebral implants to replace a damaged or degenerated natural spinal disc and to maintain sufficient stability of the disc space between adjacent vertebrae, at least until arthrodesis is achieved. Intervertebral implants can either be solid, sometimes referred to as a spacer or plug, or can define a hollow interior designed to permit bone in-growth, sometimes referred to as a fusion device or fusion cage. The interior of a fusion device may be filled with a bone growth inducing substance to facilitate or promote bone growth into and through the device to achieve a more rapid and stable arthrodesis.
Various types, shapes and configurations of intervertebral implants are known in the art. For example, one of the more prevalent designs includes intervertebral implants having a cylindrical shape and defining external threads to facilitate insertion into the disc space. As a result, reaming and tapping of the adjacent vertebral bodies is required to form a threaded passage for receiving the threaded implant. However, these techniques generally involve over-reaming of the posterior portion of the adjacent vertebral bodies, thereby resulting in excessive removal of load bearing vertebral bone which may lead to instability of the portion of the spinal column being treated. Other types of intervertebral implants have a generally rectangular configuration having planar upper and lower outer surfaces for engagement with adjacent vertebral bodies. However, the planar upper and lower outer surfaces may not adequately conform to the shape of the vertebral endplates, thereby resulting in non-uniform and inconsistent engagement between the implant and the adjacent vertebral bodies.
Additionally, most intervertebral implant designs have a predetermined, fixed height that approximates the natural height of the disc space. Insertion of an intervertebral implant having a fixed height usually requires distraction of the disc space to an insertion height somewhat greater than the natural height of the disc space. Attempts have also been made to develop various types of expandable intervertebral implants that are configured to expand along the height of the disc space. These types of expandable implants typically include multiple arms or branches having proximal end portions that extend from a fixed base, and distal end portions that remain unconnected and free to move independently of one another. A wedge is displaced between the arms to separate or splay the distal end portions of the arms apart to transition the implant to an expanded configuration defining a taper and having a maximum implant height adjacent the distal end portion of the implant. Notably, positioning of the wedge adjacent the distal end portions of the arms fails to provide support along the mid-portion of the implant to resist compression forces exerted onto the implant by the adjacent vertebral bodies. Additionally, the expansion wedge may occupy a significant portion of the inner chamber of the implant, thereby reducing the capacity of the implant to receive bone growth inducing material therein.
Moreover, some intervertebral implant designs include upper and lower bearing surfaces that are engaged against upper and lower vertebral endplates to maintain a select disc space height. These implants sometimes include teeth or other types of surface projections extending from the upper and lower bearing surfaces to aid in gripping the adjacent vertebral endplates to substantially prevent migration of the implant and possible expulsion of the implant from the disc space. However, the inclusion of teeth or other types of surface projections increases the overall height of the implant. As a result, the adjacent vertebrae must be spread apart a distance sufficient to establish a disc space height that is at least as great as the overall height of the implant, including the height of the teeth. Spreading the adjacent vertebrae apart to accommodate for the overall height of the implant may result in over distraction of the disc space. Additionally, insertion of the implant into the disc space may be impeded by the teeth or other surface projections that extend beyond the upper and lower bearing surfaces.
Thus, there is a general need in the industry to provide an improved expandable spinal implant and associated instrumentation. The present invention satisfies this need and provides other benefits and advantages in a novel and unobvious manner.